There are more kinds of organic compounds than inorganic compounds. Therefore, in organic compounds, there is a possibility to design and synthesize a substance which has various functions. From such an aspect, in recent years, electronics using an organic compound has attracted attention. For example, a solar battery, a light emitting element, a transistor, and the like using an organic compound as a functional material are typical examples.
These examples are devices utilizing an electrical property and an optical property of an organic compound, and above all, research and development of a light emitting element which uses an organic compound as a light emitting substance has remarkably progressed.
This light emitting element has a simple structure in which a light emitting layer containing an organic, compound that is a light emitting substance is provided between electrodes, and has attracted attention as a next-generation flat panel display element because of its characteristics such as a thin shape, lightweight, high response speed, and low direct current voltage driving. In addition, a display using this light emitting element has a feature that it is excellent in contrast and image quality, and has a wide viewing angle.
A light emission mechanism of a light emitting element using an organic compound as a light emitting substance is a carrier injecting type. In other words, when voltage is applied between, electrodes with a light emitting layer interposed therebetween, a hole and an electron injected from the electrodes are recombined, a light emitting substance is in an excited state, and light is emitted when the excited state returns to a ground state. As a type of an excited state, a singlet excited state (S*) and a triplet excited state (T*) are given. A statistical generation ratio thereof in a light emitting element is considered to be S*:T*=1:3.
In a compound which converts a singlet excited state into light emission (hereinafter referred to as a fluorescent compound), light emission from a triplet excited state (phosphorescence) is not observed at room temperature, and only light emission from a singlet excited state (fluorescence) is observed. Therefore, in a light emitting element using a fluorescent compound, the theoretical limit of internal quantum efficiency (the ratio of generated photons to injected carriers) is considered to be 25% based on S*:T*=1:3.
On the other hand, when a compound which converts a triplet excited state into light emission (hereinafter referred to as a phosphorescent compound) is used, internal quantum efficiency can be theoretically 75 to 100%. In other words, light emitting efficiency can be three to four times as high as that of a fluorescent compound. From such a reason, in order to achieve a high efficiency light emitting element, a light emitting element using a phosphorescent compound has been actively developed recently (for example, see Non Patent Document 1).
When a light emitting layer of a light emitting element is formed by using the above phosphorescent compound, in order to suppress concentration quenching of the phosphorescent compound or quenching due to triplet-triplet annihilation, the light emitting layer is often formed so that the phosphorescent compound is dispersed in a matrix of another substance. At this time, the substance which serves as a matrix is referred to as a host material; and the substance which is dispersed in a matrix such as a phosphorescent substance is referred to as a guest material.
When the phosphorescent compound is used as a guest material, a host material is required to have higher triplet excitation energy (an energy difference between a ground state and a triplet excited state) than that of the phosphorescent compound. It is known that CBP that is used as a host material in Non Patent Document 1 has higher triplet excitation energy than that of a phosphorescent compound which exhibits emission of green to red light, and is widely used as a host material of the phosphorescent compound.
However, although CBP has high triplet excitation energy, it is poor in ability to receive a hole or an electron; therefore, there is a problem in that driving voltage gets higher. Accordingly, as a host material of a phosphorescent compound, a substance which has high triplet excitation energy and can easily receive or transport both a hole and an electron (i.e. a bipolar substance) is required.
In addition, since singlet excitation energy (an energy difference between a ground state and a singlet excited state) is higher than triplet excitation energy, a substance having high triplet excitation energy also has high singlet excitation energy. Therefore, a substance which has high triplet excitation energy and is bipolar as described above is also effective in a light emitting element using a fluorescent compound as a light emitting substance.
[Non Patent Document 1]
    M. A. Baldo, and four others, Applied Physics Letters, vol. 75, No. 1, 4-6 (1999)